This invention relates to method and apparatus for preparing samples of solute for introduction into instruments such as gas and liquid chromatographs that are used to measure the amounts of the solutes. In these instruments, the solutes in each sample are separated from each other by introducing the sample into a stream of carrier fluid as it enters one end of a chromatographic column. In a gas chromatograph, the carrier fluid is a gas such as nitrogen; and in a liquid chromatograph, the fluid is a liquid such as methyl alcohol. Ideally, each solute emerges from the other end of the column in the form of a separate peak having a Gaussian distribution. A detector that is coupled to the output of the column produces an electrical signal corresponding to the intensity of a given characteristic of the matter contained in each peak. The carrier fluid is selected so as to have an intensity of the given characteristic that is different from that of any of the solutes of interest in a sample. When carrier fluid alone is flowing through the detector, its output signal has what is known as a "baseline value"; and when a peak of solute is flowing through the detector, its output signal forms a corresponding peak on one side or the other of the baseline. The greater the amount of a solute in the sample, the greater is the area between the signal peak and the baseline. Thus, the amount of each solute contained in a sample can be determined by integrating the difference between the signal peak and the baseline.
Where the concentrations of the solutes of interest in the liquid solvent are sufficiently large, the sample that is introduced into the column could be taken directly from the solvent, but where there are only trace amounts of the solute of interest, the amount of a solute in such a sample may be less than the minimum detectable quantity, MDQ, required for the detector to produce a useful signal. Increasing the sample size so that each solute of interest exceeds the MDQ could cause the column to malfunction or could even destroy it. By way of example, consider measuring the amount of a solute having a concentration of one part per billion, 1 ppb, in a solvent with a flame ionization detector having an MDQ of one nanogram. In order to provide this mass of solute, one milliliter of the solvent would be required and would be 100 times larger than the largest sample size generally used of ten microliters. Therefore, the concentration of the solute must be increased by at least a factor of 100 in order to produce samples of acceptable size having solutes at least equal to the MDQ. The largest sample size that can be used is often only one microliter, thus necessitating a concentration of 1000. Even greater concentrations would be desirable in order for the detector to produce signals of higher quality.
In the past thirty-five years, ever-increasing amounts of chemicals, to which living things are highly sensitive, have been introduced into the environment by industrial and agricultural practices and have found their way into drinking water. Such chemicals as hydrocarbons, chlorinated organic solvents, pesticides and dye intermediates, as well as many others, are involved. Because of the serious consequences of even small amounts of such chemicals in drinking water and because of the effect of waste water on drinking water, federal legislation was introduced in 1973 setting forth guidelines for analyzing waste water in such a manner as to detect these chemicals when they appeared with as low a concentration as one part in a billion, 1 ppb. Even lower concentrations would be of interest, but the 1 ppb was selected because this is the best that could be done with the existing methods and, as will be described, even they have serious disadvantages.